Recently, a film, a sheet, an injection molded product, a pipe, an ejection molded product, a hollow molded product and the like, which are made of a plastic, are used actively in various industrial fields. Among others, a polyethylene-based resin (ethylene-based polymer) finds widespread use, because this is, for example, inexpensive, lightweight and excellent in the moldability, stiffness, impact strength, transparency, chemical resistance and recyclability. In general, molding of the polyethylene-based resin is preformed in a molten state. However, in the case of an ethylene-based polymer alone, its melting characteristics may be insufficient, for example, in terms of flowability or the elongational viscosity may be inadequate, and in many cases, sufficient moldability can be hardly ensured or solid physical properties such as transparency and stiffness may lack.
As for the measure taken to compensate these deficiencies, a high-pressure polyethylene (HPLD) excellent in the moldability or an ethylene-based polymer differing in the molecular weight or density has been blended to improve the melting characteristics or solid physical properties (see, for example, Patent Documents 1 to 3).
Such a blend (ethylene-based resin composition) may satisfy the moldability but has a problem that reduction in the impact strength may be caused due to blending of HPLD or the molecular weight distribution or copolymerization composition distribution may be broadened to worsen the transparency.
Also, due to enforcement of the Containers Recycle Law or recent trend of resource saving, the amount of a raw material resin used needs to be reduced and from this standpoint, the demand for reduction in the wall thickness of a molded product is increasing, but in order to reduce the wall thickness, the stiffness (modulus) as well as the impact strength must be improved.
As the method for improving the impact strength, a method of reducing the density of the ethylene-based polymer is well known, but this method is disadvantageous in that the stiffness is also reduced (the polymer becomes soft), and for the purpose of reducing the wall thickness, there has been made, for example, an attempt to use a three-component blend composition obtained by further adding a specific HPLD to a combination of two kinds of specific ethylene·α-olefin copolymers differing in the density so as to enhance the moldability or transparency (see, for example, Patent Document 4).
According to this method, a polyethylene resin composition more excellent in the balance between impact strength and stiffness and also in the transparency than conventional compositions may be obtained, but the impact strength is unavoidably reduced due to blending of HPLD and furthermore, from the standpoint of stably supplying a product of constant quality in industrial level, a blend of three kinds of ethylene-based polymers is considered to be economically disadvantageous as compared with conventional compositions.
On the other hand, as to the method for improving the moldability, an attempt to introduce a long-chain branched structure capable of increasing the melt viscosity into an ethylene-based polymer has been made, but since the design for optimizing the long-chain branched structure is insufficient, reduction in the strength or transparency still cannot be avoided and the improvement level of moldability is yet low (see, for example, Patent Documents 5 to 8).
The polyolefin produced using a metallocene catalyst for olefin polymerization has a high uniformity in the polymer molecular structure such as molecular weight distribution or copolymerization composition distribution and is excellent in various mechanical properties such as impact strength and long lifetime and therefore, the amount used thereof is recently increasing. The metallocene-based polyolefin is excellent in various mechanical properties but, because of its narrow molecular weight distribution, is poor in the characteristics important to molding of a polyolefin, such as melt tension and melt flowability, and cannot satisfy sufficient performance in terms of mold processing.
As the method for improving the insufficient mold processability of the metallocene-based polyolefin, a method where the melt viscosity is increased by introducing a long-chain branch into a polyethylene by a polymerization reaction using a specific metallocene complex and the flowability or melt tension is thereby improved, is well known (see, for example, Patent Document 9). With respect to the specific metallocene complex for introducing a long-chain branch, a method using a bridged bisindenyl complex (see, for example, Patent Documents 10 to 13) or a constrained geometry half-metallocene complex (see, for example, Patent Document 14) is well known, but the long-chain branch obtained by such a method does not develop as in the structure of HPLD, and the improvement of melt viscosity of the polymer is not sufficient.
Also, as to the method for introducing a long-chain branch by a metallocene complex other than those described above, a method for producing a long-chain branch-containing polyethylene by using a bridged metallocene catalyst having a specific structure is known (see, for example, Patent Document 15), and specifically, an example of simultaneously using two kinds of metallocene complexes of dimethylsilylbis(pentadienyl)zirconium dichloride and diphenylmethylene(cyclopentadienyl)(fluorenyl)zirconium dichloride is shown. As the improved technology thereof, furthermore, a catalyst where the above-described two kinds of complexes are co-supported on the same support (see, for example, Patent Documents 16 to 18), and a catalyst where the kind of the complex combined is improved (see, for example, Patent Documents 19 and 20), are known. In these methods, it is supposed that one kind of a complex produces a polymer having a polymerizable double bond at the terminal (so-called macromer) and another complex is a complex excellent in the copolymerizability and copolymerizes with the macromer to form a long-chain branched structure.
Such a method may achieve introduction of a branched structure that is somewhat more developed than the long-chain branched structure by the conventional metallocene complex, but the level of development is still insufficient or those two kinds of metallocene complexes greatly differ in the copolymerizability of a comonomer, giving rise to a problem that the copolymerization composition distribution of the produced polymer may be broadened or the mechanical properties may deteriorate in association with production of a low-melting-point polymer.
Patent Document 21 has reported that when homopolymerization of ethylene is performed according to solution polymerization by using an asymmetric metallocene in which a cyclopentadienyl group and an indenyl group are carbon-bridged and methylaluminoxane, a branch-containing polyethylene can be produced, but the carbon number indicative of the branch length is from 1 to 20. Thus, the branch length is too short to exert an effect of improving mold processability as a long-chain branch, failing in exhibiting strain hardening of the elongational viscosity.
Also, Patent Document 22 has reported polymerization of propylene by a polymerization catalyst prepared by combining an asymmetric metallocene compound in which a cyclopentadienyl group and an indenyl group, each having a specific substituent, are bridged, with methylaminoxane, but this document is silent on the possibility of producing a long-chain branch when the polymerization above is applied to polymerization of ethylene, and the effect of improving the mold processability cannot be expected.
Furthermore, Patent Document 23 has reported a catalyst system capable of producing an ethylene polymer and an ethylene/butene copolymer, which are useful as a macromonomer, by using, out of asymmetric metallocenes in which a cyclopentadienyl group and an indenyl group are silicon-bridged, a metallocene having a methyl group on the 2-, 4- and 7-positions of the indenyl group and a modified clay compound, but the number of terminal double bonds of the polymer is small, and this document is silent on the possibility of producing a long-chain branch by this catalyst alone.
Recently, the present inventors have reported in Patent Document 24a method for producing an ethylene-based polymer improved in the mold processability by using a supported catalyst for olefin polymerization comprising, as an essential component, out of asymmetric metallocenes in which a cyclopentadienyl group and an indenyl group are bridged by a bridging group, a specific asymmetric metallocene having no substituent except for the bridging group on the cyclopentadienyl group and having a hydrogen atom or a specific substituent on the 3-position of the indenyl group. According to this method, an ethylene-based polymer having a large degree of strain hardening of the elongational viscosity is obtained and therefore, the mold processability is improved as compared with the conventional long-chain branched polyethylene, but since the long-chain branching index does not reach that of a high-pressure low-density polyethylene, more improvement of the long-chain branched structure is demanded.
Under these circumstances, it is required to solve the problems of the conventional ethylene-based resin composition and develop a polyethylene-based resin composition excellent in the moldability and capable of producing a molded product excellent in the balance between impact strength and stiffness as well as in the transparency. Furthermore, in order to solve the problems of a long-chain branch-containing polyethylene in conventional techniques and improve the mold processability of a metallocene-based polyethylene, it is required to develop an ethylene-based polymer having introduced thereinto a sufficiently large number of appropriate-length long-chain branches, an olefin polymerization catalyst excellent in the introduction of long-chain branch, and a production method of an olefin-based polymer.